In a rectifying circuit contained in a DC power supply apparatus of related art, since an input current inputted into the DC power supply apparatus from an AC power supply can not be rectified synchronously with the voltage of the AC power supply, the power factor of the power supply is low and the reactive power amount is large. That is, there arises a problem that the electric power utilization factor is low. The reactive power component is the power returned to the power supply side without being consumed by a load side connected via the DC power supply apparatus, among the power supplied from the power supply. The presence of the reactive power component corresponds to a state that the efficiency of each of the electric power generation and the electric power transmission is low when seen from the power supply side and a state that the electric power supplied from the power supply is not utilized effectively when seen from the load side. Further, the DC power supply apparatus of the related art has a problem that the waveform of the input current inputted from the AC power supply deforms from the sinusoidal waveform, that is, the input current contains much harmonic current component. Thus, there arises a problem that the operation of other apparatuses connected to the same power supply system is interfered and the power transmission facility is damaged. Such the problems are required to be improved according to the International Standard (IEC61000-3) etc. As the measures for the improvement, a DC power supply apparatus is used which performs the PWM control by using semiconductor switching elements to thereby improve the power factor of a power supply, suppress the harmonic current of the power supply and adjust the DC output voltage (see JP-A-2001-286149 (pages 11 to 12, FIGS. 1 to 4)).
Further, according to a recent tendency of energy saving, an inverter circuit for driving a motor etc. uses a Schottky Barrier diode (SBD) made of silicon carbide (SiC) as a power semiconductor module to thereby reduce steady loss due to the voltage drop of the power semiconductor module and to thereby increase the switching speed (turn-on speed) of the power semiconductor module to reduce the switching loss, so as to reduce the loss and the heating amount of the power semiconductor module while almost maintaining the current driving efficiency of the motor etc. (see JP-A-2008-92663 (pages 3 to 4))
In the related-art DC power supply apparatus connected to the AC power supply for improving the power factor of the power supply and suppressing the harmonic current of the power supply, the switching is performed by the switching frequency of about 20 kHz to 25 kHz. In this case, since a large current ripple occurs due to the switching operation on the input current from the AC power supply, there arises a problem that a portion for removing the current ripple component, that is, a filter circuit is required. Further, when the switching frequency is low, since the time control of the current and voltage according to a PWM control becomes rough, there arise problems that the input current from which the current ripple has been removed can not maintain the sinusoidal waveform and is distorted and further a phase shift occurs between the input current and the voltage of the power supply. Furthermore, since the filter circuit for removing the large current ripple component largely influences on the phase of the input current, the waveform of the input current is deformed or distorted. In view of these problems, it is desired to set the harmonic current of the power supply to 0 and set the power factor of the power supply to 1 according to a theoretical design, that is, according to the control by a control circuit.
As a method for solving the problems, related-art discloses a method of increasing the switching frequency of the DC power supply apparatus to thereby finely perform the time control of the PWM control. When the switching frequency is increased, the current ripple becomes small. Thus, the current ripple component can be removed by a filter circuit which scarcely influences except for the removal of the current ripple component. Further, the input current of the sinusoidal waveform having small distortion can be generated due to the fine time control of the PWM control. As a result, the harmonic current of the power supply can be reduced and the power factor of the power supply can be improved, so as to become close to a theoretical design.
However, in the case of increasing the switching frequency of the DC power supply apparatus to be connected to the AC power supply, even when the voltage of the AC power supply is applied so as to follow the high-speed switching operation of the semiconductor switching elements, there is a problem that there is no rectifying element having a high withstanding voltage and current which can prevent the dielectric breakdown thereof.
Further, when the switching frequency is increased in the constituent components of the related art, the loss of each of the semiconductor elements, that is, the rectifying elements and the semiconductor switching elements on a path for flowing the current from the DC power supply apparatus becomes large. Thus, there arise problems that the efficiency of the DC power supply apparatus reduces and the semiconductor elements can not withstand heat generated by the loss and are burnt.
Further, when the switching frequency is increased, since the amount of heat generation increases due to the increase of the loss of the semiconductor elements of the DC power supply apparatus, there arises a problem that the size and the cost of a cooling apparatus increases.
Furthermore, since high-frequency noise is generated due to the high-speed switching operation of the semiconductor elements of the DC power supply apparatus, there arise problems that another apparatus except for the DC power supply apparatus is erroneously operated or the DC power supply apparatus itself is erroneously operated.